Abstract

The Port Cygnet alkaline complex consists of small scatteredconcordant and discordant intrusions of Cretaceous age emplaced inJurassic dolerite and Permian fluvioglacial sediments in the vicinityof Port Cygnet, southern Tasmania.The alkaline rocks are miaskitic and fall into two main groups:one is a group of slightly oversaturated syenites with a porphyritictexture rich in phenocrysts of oligoclase (syenite porphyries), andthe other consists of nepheline syenites, characteristically with aflow structure and containing phenocrysts of euhedral sanidine crystals,with feldspathoids in the groundmass, and occasionally as phenocrysts.These are the sanidine rocks or sanidine porphyries.The primary magma was a potash-rich upper mantle or lower crustal,partial melt (of a mafic alkali parent) probably equivalent ultimatelyto a tinguaite which began crystallizing in a magma chamber withsubsequent successive injections of magma containing coarser sanidinephenocrysts. . This gave rise to the nepheline syenites.The intrusion of the potassic magma was accompanied by volatileswith evidence of the presence of water, carbon dioxide, sulphurdioxide and hydrogen sulphide, demonstrated by the presence of bothprimary and secondary reaction mineral products.The porphyritic syenites seem to have been derived by the mixingof a potassic upper mantle or lower crustal partial melt, with acrustal partial melt of the feldspathic matrix of an amphibolitefollowed by subsequent initial crystallization at depth beforeinjection into the country rocks.The model is based on microscopic and textural characteristics,chemical compositions, mineralogy and initial strontium isotope ratios.Variation of the syenite porphyries due to addition ofrecrystallized amphibole from the parent amphibolite formed a horn-blende porphyry. Fractionation of the syenitic melt has resultedin a group of mixed feldspar phenocryst rocks showing potash feldsparreacting with plagioclase phenocrysts in a brown matrix.Assimilation of mineralized carbonates by the magma has producedone rock unique in having phenocrysts of spessartite and epidote.Sulphur from the country rocks was mobilized by this intrusion.At Regatta Point, on the western shore of Port Cygnet, the.potassic magma reacted with Jurassic dolerite to form hybrid rocks.These were most likely produced within the temperature range of710 °C to 810 °C and a log £0 2 in the range -11 to -13.The alkaline intrusion was probably associated with the break-upof Gondwanaland.

Copyright 1984 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

5 maps in pocket in v. 2. Includes bibliography. Thesis (Ph.D) - University of Tasmania, 1984